1. Field of the Invention
This invention relates to a rear wheel turning system for a vehicle, and more particularly to a rear wheel turning system for a vehicle in which the rear wheels are turned by an actuator which is driven by a battery mounted on the vehicle.
2. Description of the Prior Art
Though, in a conventional two-wheel-steered vehicle, only the front wheels are turned when the vehicle is steered, in four-wheel-steered vehicles which have been recently put into use, both the front wheels and the rear wheels are turned when the vehicle is steered. That is, such a four-wheel-steered vehicle is provided with a four-wheel steering system which comprises a rear wheel turning system for turning the rear wheels and a front wheel turning system for turning the front wheels.
In one type of known rear wheel turning system, the rear wheels are turned by an actuator which is driven by the engine output power of the vehicle, and in another type of known rear wheel turning system, the rear wheels are turned by an actuator which is driven by a battery mounted on the vehicle. For example, the rear wheel turning system disclosed in U.S. Pat. No. 4,732,231 comprises a hydraulic power cylinder which turns the rear wheels with the hydraulic pressure provided thereto from a hydraulic pump. The pressure is controlled by means of a control valve, and the hydraulic pump is driven by the engine.
In the case of a rear wheel turning system in which the rear wheels are turned by an actuator which is driven by the engine output power, the rear wheels can be turned only when the engine operates, and when the engine stalls, the rear wheels can no longer be turned. In order to make the vehicle safe when the rear wheel turning system becomes uncontrollable, there is generally provided a rear wheel neutralizing means which comprises a centering spring and urges the rear wheels toward the neutral position, i.e., the straight-ahead position. When the engine stalls, the rear wheel neutralizing means returns the rear wheels to the straight-ahead position, and thereafter the vehicle is made to behave as a two-wheel-steered vehicle.
However, the rear wheel turning system encounters the following problems.
For example, in the rear wheel turning system disclosed in the above identified patent publication, the rear wheels are turned in a direction opposite to the turning direction of the front wheels when the vehicle is running at a low speed in order to improve the heading performance and in the same direction as the turning direction of the front wheels when the vehicle is running at a high speed in order to improve the driving stability. When the engine stalls while the vehicle is travelling at a low speed, for instance, while it is being driven into or out of a garage, the vehicle is not generally braked at once and is allowed to run by inertia for a while. Since the rear wheels are returned to the straight-ahead position as soon as the engine stalls, the turning radius will become larger and the vehicle will follow a larger arc, which can cause an accident. Further, when the engine stalls while the vehicle is running downhill at a certain high speed, the driver will not generally restart the engine as soon as the engine stalls but will allow the vehicle to coast for a while with the engine in a stalled state. In such a case, the rear wheels are returned to the straight-ahead position as soon as the engine stalls. Accordingly, the attitude of the vehicle will change and the driving stability will deteriorate as the vehicle coasts.
On the other hand, in the rear wheel turning system disclosed in U.S. Pat. No. 4,645,025, the rear wheels are turned by an electric motor which is driven by an onboard battery. In the case of a rear wheel turning system which is driven by a battery, the rear wheels can be turned even after the engine stalls, and accordingly, the aforesaid problems inherent to a rear wheel turning system in which the rear wheels are turned by the engine output power can be avoided. However, there arises another problem in that the battery can die if the driver keeps the engine in a stalled state for a long time.